stability of heun method

Hey,
I'm trying to determine the region of stability for the Heun method and whether it is A stable. I've looked at the examples on wiki here Stiff equation - Wikipedia, the free encyclopedia and Runge here. By plugging in the test equation y' = zy where k is complex I've simplified the Heun algorithm from

then when I insert , my result simplifies to

to judge from the wiki article, the stability region is then the area described by

Am I on the right path at all? How does this relate to A stability? Appreciate any help.